1. Field of the Invention
This present invention relates generally to the field of integrated circuit connectivity and, more specifically, to the field of wire bonding integrated circuits using copper metalized bond pads.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In today""s complex computer systems, speed, flexibility, and reliability in timing and control are issues typically considered by design engineers tasked with meeting customer requirements while implementing innovations which are constantly being developed for computer systems and their components. Computer systems typically include a variety of electrically interconnected integrated circuit (I/C) packages which perform a variety of functions, including memory and processing functions. These I/C packages typically include numerous bond pads that interface with external connectors, used to join the assorted circuits together. Typically, the external connectors that interface with the bond pads are either wires or solder balls.
Until recently, bond pads typically have been formed from aluminum due to that metal""s relatively low resistivity and good current-carrying capabilities. Additionally, aluminum adheres well to silicon dioxide, is available in high purity, and has a naturally low contact resistance with silicon. While gold and copper are more conductive than aluminum, both metals are typically used less frequently for reasons not related to conductivity, such as their high susceptibility to contamination or oxidation.
Recently, however, there has been renewed interest in copper metalization due to the superior conductivity and scalability of copper. In particular, the reductions necessary for 0.25 xcexcm and smaller scale devices have spurred interest in copper as an alternative to aluminum. For example, some vias or other interconnect structures may be too narrow to form efficiently from aluminum. Other advantages of copper, include its superior conductivity, good step coverage during deposition processes, resistance to electromigration, and low temperature deposition.
A disadvantage of copper, however, is its susceptibility to oxidation. The resulting layer of surface oxidation is unsuitable for forming electrical contacts and is difficult to weld. The susceptibility of copper bond pads to oxidation is particularly problematic since the oxidation can inhibit the wire bonding of I/C dies and substrates. The technique of wire bonding includes bonding a thin wire to a chip or die bond pad, spanning the wire to the inner lead of the package lead frame, and bonding the other end of the wire to the associated inner lead bond pad. Bonding may be performed by a variety of means including thermocompression, thermosonic, and wedge (or ultrasonic) bonding. The process of wire bonding requires not only precise wire placement but also good electrical contact at both ends. Aluminum and gold wire have typically been used for wire bonding due to their acceptable conductivity and scalability. Gold wire is of particular interest in ball bonding as it does not need a cover gas after ball formation or flame off.
As noted, however, oxidation on copper inhibits useful bonds from being formed on copper bond pads. Current techniques for addressing this problem include depositing a layer of nickel upon the layer of copper and then coating the layer of nickel with a layer of gold. The nickel layer serves as a barrier layer, preventing migration of the copper through the gold. If allowed, such migration would lead to copper oxidation, resulting in poor solderability and increased contact resistance. The nickel barrier layer is typically electrolessly plated. The selective nature of electroless plating allows targeted plating of the area of interest as opposed to non-selective techniques such as electrodeposition. Additionally, electroless plating is typically preferred to the use of immersion solutions when plating onto thin layers since immersion solutions typically consume the underlying material and may thereby create adhesion issues with underlying layers. Electroless plating, however, does not degrade underlying thin layers, and may also plate thicker layers of material, offering protection against the ultrasonic energy of the wirebonder.
While the use of a gold cap allows wire bonding to the copper bond pads, the necessity of the gold layer presents distinct problems in addition to the need for a barrier layer. For example, gold is expensive and its supply is often unpredictable, leading to elevated costs. It would be preferable, therefore, to be able to rely solely upon a layer of nickel, without the gold cap, both in terms of cost and in terms of reducing the number of production steps. The present invention may address one or more of the concerns set forth above.